Spring has sprung! Five things you need to know about the vernal equinox

Happy first day of Spring! Winter has officially ended across the northern hemisphere with March 20 marking the first day of the new season. To celebrate the day in its own way, Google's latest Doodle sees a hibernating mouse awake from a slumber to collect a newly-grown flower.

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"We use the equinox to mark the change of seasons, as the balance of light shifts to make for longer days or nights," explains Google. "It usually means it's time to hunker down for colder seasons, or time to rise and shine for warmer ones."

Officially known as the vernal equinox, the start of Spring is one of two equinoxes to happen each year. The second occurs in the Autumn, on September 22, 2017. Aside from quirky animations, the day indicates a returning to warmer weather and a longer period of daylight in the northern hemisphere. Here are four things you might not know about the changing of the seasons.

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What causes an equinox?

An equinox is an astronomical event that occurs due to the Earth's tilt on its axis. For half the year, the northern hemisphere gets more daylight while for the other half the southern hemisphere has "longer" days.

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Equinoxes occur when the Earth is at the midpoint of its axis. Both the equinoxes happen around the same time each year – usually within three days of the previous year's solstice. The weekend after the equinox is typically when the clocks change in the UK. They 'spring' forward an hour to make evenings lighter. Daylight Savings Time lasts in the UK until October 29, when the clocks 'fall' back an hour.

The 'equal night'

Nasa explains that equinox literally means "equal night" in Latin. On both of the year's equinoxes, there are almost equal amounts of day and night in both of the world's hemispheres.

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"The equinox marks the onset of Spring with a transition from shorter to longer days for half the planet, along with more direct sunlight as the sun rises higher above the horizon," Nasa says.

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The solstice

If you consider the Earth's axis in a similar way to a clock face, the equinoxes happen when the Earth is at 12PM and 6PM. When the Earth reaches 3PM and 9PM for each of the two hemispheres, the Earth is at the most titled points on its axis.

When this occurs, Earth gets its shortest and longest days and this is what's known as the summer and winter solstices. Solstice, in Latin, can be translated to mean "sun stands still". Nasa adds: "These days are our longest and shortest days, and mark the change of seasons to summer and winter."

The equinox from space

On Earth, the most obvious signs the Spring equinox has altered the seasons is fairly obvious: days change length and for the northern hemisphere and the temperature (in theory) warms up. However, the equinox can be viewed another way – through Nasa's telescopes and satellites.

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The video above shows what the changing of the seasons looks like off planet and the movement of the Earth on its uneven axis.

What's the equilux?

While the equinox is known for having equal amounts of day and night, it is not an exact split. Instead, the equilux is the day when there's an exact 12 hour split, by the minute, of day and night. The equilux in 2017 actually took place a few days before the equinox on March 17.

Moon phases during Spring Equinox 2017

March 20 was not just the day of the vernal equinox. It was also the last quarter moon. It came three weeks after the previous new moon and eight days before the next new moon, set to be visible on March 28 at 2:57 UTC.